The shell_widget is the pop-up shell to pop down, and the enable_widget
is the widget that was used to pop it up.

XtCallbackPopdown calls XtPopdown
with the specified shell_widget and then calls XtSetSensitive to resensitize
the enable_widget.

If a shell name is not given, MenuPopdown calls XtPopdown
with the widget for which the translation is specified. If a shell_name
is specified in the translation table, MenuPopdown tries to find the
shell by looking up the widget tree starting at the parent of the widget
in which it is invoked. If it finds a shell with the specified name in the
pop-up children of that parent, it pops down the shell; otherwise, it
moves up the parent chain as needed. If MenuPopdown gets to the application
top-level shell widget and cannot find a matching shell, it generates
an error.